1. Field of the Invention
This invention relates to aqueous polyurethane dispersions and more particularly to crosslinked polyurethane dispersions.
2. Description of the Prior Art
Water dispersable polyurethanes are well known and have achieved acceptance in a variety of applications. U.S. Pat. No. 2,968,595 discloses the emulsification of free isocyanate containing prepolymers in a solution of diamine and water with the aid of detergents and under the action of powerful shearing forces. The free isocyanate reacts with the water and diamine to "chain extend" the polyurethane polymer to form dispersed particles of thermoplastic polyurethane polymer. As is taught by U.S. Pat. No. 2,968,595, the resultant emulsion can be cast and dried to form a thermoplastic polyurethane film or coating. The emulsions prepared according to the 2,968,595 patent have the disadvantage that in forming the emulsion a detergent must be used. Because the detergent is not volatile and inherently contains hydrophilic groups, the physical and chemical properties of the resultant coating are adversely affected by the detergent. In addition, insufficient shearing force often results in an unstable emulsion. Further, the emulsion cannot be produced in conventional reaction kettles because of the high shearing forces required to form a homogeneous stable emulsion.
The prior art, particularly U.S. Pat. No. 3,479,310, has suggested and taught that a fully chain extended, isocyanate-free polyurethane, having an internal salt along the chain, be prepared and dispersed in water. However, the chain extension results in a high viscosity, high molecular weight product which is difficult to disperse in water. The dispersion in water requires thinning the polymer in organic solvents to achieve the requisite viscosity for emulsification, thus requiring removal of organic solvents, by steam distillation or the like, to achieve the advantages of an aqueous system.
Further, the synthesis-dispersion technique provides a coarse dispersion and requires a high percentage of internal salt groups to effect a stable dispersion. Because of the hydrophilic nature imparted to the polymer by the high percentage of internal salt groups, coatings formed from dried cast films are moisture sensitive.
The presently preferred and accepted system for preparing ionic polyurethane dispersions is by synthesizing polymers that have free acid groups covalently bonded to the polymer chain or backbone. Neutralization of these acid groups with an amine, preferably a water soluble monoamine, yields a water reducible polymer. Preferably the acid group is a carboxylic acid group but phosphorous or sulfur based acid groups may also be used. The compound bearing the acid group must be carefully selected so that the isocyanate groups, necessary to form the polyurethane, do not react therewith. This is accomplished, typically, by selecting a compound which has a sterically hindered acid group having reduced reactivity with isocyanate groups. Exemplary of the use of the sterically hindered acid group principle are the teachings and the disclosure of U.S. Pat. No. 3,412,054, incorporated by reference herein, wherein 2,2-hydroxymethyl substituted carboxylic acids are reacted with organic isocyanate groups without significant reaction between the acid and isocyanate groups due to steric hinderance of the carboxyl groups by the adjacent alkyl groups. This approach provides the desired carboxyl containing polymer with the carboxylic group being neutralized with a tertiary monoamine to provide an internal quaternary ammonium salt and hence, water reducibility.
Since fully chain extended high molecular weight, isocyanate free polyurethanes tend to have very high viscosities even at elevated temperatures, it is not practical to disperse them in water after the chain extension reaction has been completed. The preferred method is to make an isocyanate-terminated prepolymer containing the solubilizing amine salt of the carboxylic acid and disperse the prepolymer in water while the prepolymer is at a workable viscosity. Once the prepolymer is dispersed in the water, chain extension to high molecular weight takes place in the dispersed droplets by reaction with water or with any suitable chain extender that is present in the water, for example, a water soluble diamine.
It is well known to those of ordinary skill in the art that in the preparation of fully reacted polyurethane polymers that are to be cast in solution as films or coatings, that the use of reactants with a functionality of greater than two is very hazardous since they promote branching and unless the branching is controlled will quickly result in the gelation of the growing polymer chains into an intractable mass. Knowing the concentration and functionality of each of the reactants yields the predictability of knowing at what point during the reaction that the growing polymer will become an insoluble continuous network or gel, i.e., crosslinked. These calculations for the gel point are well defined in such textbooks as Flory's Principles of Polymer Chemistry. They may be found in Chapter IX entitled "Molecular Weight Distributions in Nonlinear Polymers and the Theory of Gelation", pages 347-361, incorporated herein by reference.
While numerous cross-links in a coating have obvious advantages in terms of solvent resistance and heat resistance, it is generally not possible to prepare such a film or coating without resorting to two component systems with all their inherent disadvantages or to polymers with built-in reactive sites that require heat or radiation for crosslinking. Although numerous urethane coatings have been commercialized using the two component systems and built-in reactive sites, a desirable improvement in the state of the art would be to achieve a product comparable to these systems without their attendant disadvantages.
Thus, in accordance with the present invention a process for preparing crosslinked but coalescible dispersed polymer particles is provided. Further, these particles, being cast from an aqueous dispersion, are film forming at room temperature, thus requiring no heat that can damage sensitive substrates.
Further, in accordance with the present invention, the advantages of a fully reacted system without highly reactive and toxic co-reactants are provided. Further, the attendant advantages of an aqueous system are provided by the present invention.